1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method, both designed to convert a signal to a progressive signal the signal composed of an interlaced signal generated by the 3-2 pull-down process or the 2-2 pull-down process and an ordinary signal interlaced at the rate of, for example, 60 fields/sec.
This application cams priority of Japanese Patent Application No. 2003-311627, filed on Sep. 3, 2003, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
Standard television signals, such as NTSC signals and high-definition signals, are interlaced signals. FIG. 1A shows the scanning lines for an interlaced signal. FIG. 1B depicts the scanning lines for a progressive signal. FIG. 1C shows a progressive signal obtained by converting an interlaced signal through scanning-line interpolation. In FIGS. 1A, 1B and 1C, “o” indicates a scanning line and “x” indicates a scanning line interpolted.
In FIGS. 1A, 1B and 1C, arrow V represents the vertical direction, while arrow t represents the time axis. As illustrated in FIG. 1A, each frame of the interlaced signal consists of two fields that are dislocated from each other in the time axis and the vertical direction. By contrast, the progressive signal is free of field dislocation as seen from FIG. 1B. The interlaced signal may have an interlace disturbance, such as line flicker, if it has a frequency component that is prominent in the vertical direction of image. The progressive signal does not have an interlace disturbance.
There is a method of eliminating the interlace disturbance. In the method, any scanning line extracted in the interlace process is interpolated by the surrounding scanning lines, as is illustrated in FIG. 1C. This method is known as “progressive transform” or “double-density transform.”
The scanning-line interpolation performed in the progressive transform is motion-adaptive interpolation. That is, as shown in FIG. 2, the inter-field interpolation is carried out, generating a new scanning line. More precisely, for a still picture, an average value of signals PA and PB representing two field pixels adjacent in the horizontal direction is obtained, generating a signal PQ that resets a new pixel x. For a moving picture, on the other hand, the intra-field interpolation is carried out, generating a new scanning line. An average value of signals PC and PD representing two field pixels adjacent in the vertical direction is obtained, generating the signal PQ that represents the new pixel x. If the image is a still picture, the progressive transform can provide an image that has little folding distortion and high resolution. If the image is a moving picture, however, the progressive transform results in an image that has a conspicuous folding distortion and very low resolution.
Assume that the input signal that should be subjected to the progressive transform may be an interlaced signal generated by the 3-2 pull-down process or the 2-2 pull-down process. Then, a method other than the motion-adaptive interpolation may be used. In this case, the progressive transform can provide a high-quality image even if the image moves. Note that the 3-2 pull-down process is a frame-rate conversion shown in FIG. 3. This process is used as a method of converting progressive signals A, B, C, . . . , such as 24-frames/sec film data, to interlaced signals a, a′, a, b′, b, c′, c, c′, . . . of 60-fields/sec, NTSC scheme. In FIG. 3, the prime (′) shows whether the signal pertains to an even-numbered field or an odd-numbered field. Also note at the 2-2 pull-down process is such a frame-rate conversion as illustrated in FIG. 4. The 2-2 pull-down process is employed as a method of converting progressive signals A, B, C, . . . , such as 30 frames/sec film data, to interlaced signals a, a′, b, b′, c, c′, . . . of 60-fields/sec, NTSC scheme.
As seen from FIG. 3, the original image, i.e., one-frame image, is divided into two or three fields in the 3-2 pull-down process. As can be understood from FIG. 4, the original image, i.e., one me image, is divided into two fields in the 2-2 pull-down process. Thus, if the 3-2 or 2-2 pattern of the input signal acquired by the 3-2 or 2-2 pull-down process is known, the input signal can be converted to a progressive signal in the 3-2 or 2-2 pull-down process, by performing intra-field interpolation on only the adjacent fields generated from one and the same frame. This can be accomplished no matter whether the image is a still picture or a moving picture. The intra-field interpolation is a process that is different from the inter-field interpolation shown in FIG. 2. Nonetheless, it is similar in that the signal PA for the preceding field or the signal PB for the following field is used as signal PQ that represents a new pixel, thereby to generate a new scanning line. The intra-field interpolation can, therefore, provide images that have little folding distortion and high resolution.
An ordinary 60-fields/sec signal may be inserted by edition into an interlaced signal generated by the 3-2 pull-down process or 2-2 pull-down process. Such an interlaced signal tat contains a 60-fields/sec signal to a progressive signal cannot be converted to an optimal progressive signal by means of intra-field interpolation. This is inevitable because in the ordinary 60-fields/sec signal no field generated from an image exists between two adjacent fields generated from the same image, particularly when the ordinary 60-fields/sec signal represents a moving picture. Consequently, this interlaced signal represents, but a low-quality image. Assume that an interlaced signal obtained by the 2-2 pull-down process and containing a 60-fields/sec signal representing a round object is converted to a progressive signal, as is illustrated in FIG. 5. Also assume that the intra-field interpolation has been performed in the 2-2 pull-down process. Then, the resultant image will include two identical images of the round object, which overlap each other, simply because the ordinary 60-fields/sec signal represents a moving picture of the round object. Obviously, the image is much degraded in quality.
This problem may be solved by the technique disclosed in Japanese Patent Application Laid-Open Publication No. 2000-78535. This technique is to convert a signal into a desirable progressive signal even if the signal consists of an interlaced signal obtained by the 3-2 or 2-2 pull-down process and an ordinary 60-fields/sec signal, without degrading the quality of image. In the technique, one of three signals, which has the smallest absolute value is selected and used as a motion signal K. The three signals are: (i) a succeeding intra-field interpolated signal that is a scanning-line signal identical with the interpolated scanning line for the field succeeding the field of interest in time; (ii) a preceding intra-field interpolated signal that is a scanning-line signal identical with the interpolated scanning line for the field preceding the field of interest in time; and (iii) an inter-frame matching signal that represents the absolute value of the difference between the succeeding intra-field interpolated signal and the preceding intra-field interpolated signal. The motion signal K is applied to obtain an optimal signal. More specifically, the mixing ratio between an intra-field interpolated signal and an intra-field interpolated signal for the field of interest is changed in accordance with the motion signal K. The “intra-field interpolated signal” is composed of the succeeding intra-field interpolated signal and the preceding intra-field interpolated signal. The “intra-field interpolated signal for the field of interest” has been generated by adding the two scanning lines above and below the interpolated scanning line for the field of interest, respectively.
In the technique disclosed in Japanese Patent Application Laid-Open Publication No. 2000-78535, the pixel data items of different fields, i.e., pixel data items acquired at different times, are compared. Any pixel data item pertaining to a moving image inserted therefore changes in value with time. Hence, the intra-field interpolation is predominant. In consequence, any image part other than the inserted image part which can be converted into a desirable progressive signal without degrading the image quality, will inevitably be degraded in quality.